FAT MAN

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PSLimo

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Hey Guy's,

I've been wanting to build a Fat Man for a while now. I decided to build a small 8" diameter one first and then go 1/4 or 1/2 scale (15" or 30" diameter). Will be easier to see how it performs small scale. The 8" diameter was chosen because that's the largest plastic egg I could find.

Keeping it very simple I put 3.9" bodytube up the middle of the egg and will blow everything motor and all out the back using a coupler tube.

The reason I'm posting is I'd like to see any ideas anyone has on a shell I can use on a scaled up version now that you see the basic design in the pictures below. Can't find any blow molds or eggs big enough. Also want to stay away from shaping foam.

Any ideas???

Phil

fat_man_atomic_bomb_by_silentnight745.jpg

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I don't know of anything off the shelf like that, but maybe take a large balloon and inflate it to the diameter you need. You could then papier-mâché the balloon, and once it dries cut it in half and use it as a two piece mold for fiberglass.
 
Wow, Phil; this is awesome! ...and here I thought you were getting out of rockets!

Closest thing I can think of is those big plastic Halloween pumpkins that used to be flown around here. They have the right general shape, but would have to be faired/smoothed in order to look more like a smooth egg and less like a big pumpkin! They are about 21" in diameter; I have several if you need one.

Cool project!!!!

-Eric-
 
Wow, I love these kind of projects where you have to figure out how to make the parts !! I'll bounce this idea off you. Use a balloon---fill it partially with water---you'll have to play with the amount. ---fill the rest with air and hang it upside down. The weight of the water should give you the elongated look you need---that's why you'll have to play with the water/ air ratio. Once you have the shape you want, paint it with several coats of finishing resin. The stuff dries slow so it might take a few days for this process. This will give you an aero shell. Slowly deflate the balloon and carefully remove it. The aero shell is delicate but you can now put fiber gass strips inside to strengthen it . Should be pretty sturdy and light. ------H
 
Hey Guy's,

Good to hear from you Eric. I'll be at Bong in September to see my Mega Nuke fly on a 6 grain 75 mm motor.

Thanks for the ideas. I've used a balloon to make a nosecone for my full scale Javelin 148. The pictures are below. I tried to glass a full size balloon in a prototype for a 15" diameter Fat Man and there just wasn't enough rigidness in the balloon to keep shape.

Still thinking...

Phil

IMG_1772.jpg

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Man, you scratch builders are frickin' magicians ! How about some expandable foam filling the area between the shell and the tube ? Too much of a weight penalty ?
 
Looking at the shape of the bomb, Peanut M&M's come to mind. If you could find a store display of a big Peanut M&M that might work, that is if it's perfectly round.
 
No worries on strengthening the prototype's shell with foam. It's more rigid than most nosecones.

I'm talking about forming a shell on a scaled up Fat Man. The peanut M & M is the right oblong shape for sure, just can't find one smooth. Same problem with the plastic pumpkins. A lighter shell is a good thing being all the weight you need in the nose to make it stable. Fortunately, a rocket buddy that posts as markkoelsch here is kind enough to do the RockSim work on my designs making sure they're safe to fly. Here's the 1/4 scale Fat Man Sim.

Phil

FatMan1.jpg
 
Im surprised that the CP is that far back. I have always wanted to do a bomb style shape but never figured out how to do it in Open Rocket.
 
It takes a lot of weight in the nose and those draggy box/fins are set behind the aft of the rocket which doesn't help stability.

I've made quite a few different bomb's fly. One of my favorites was the 7.5" diameter CBU-97 below I called "Da Bomb".

thebomb.jpg
 
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It's looking more like the Fat Man.

I thought about "H's" putting water in the balloon comment. What if I put something inside the balloon to strengthen it enough to glass over and still retain it's shape? "Fix a Flat" came to mind. Pretty much quick drying latex paint to coat the inside of the balloon. Roll it around until it hardens. Problem is, what if the balloon bursts. I'd have a paint bomb.

Plaster of Paris might work. Pump some in the balloon, blow it up and roll it around until it coats the inside and hardens. Spray it with a release agent and glass it. Cut the bottom out to bodytube size and it shouldn't be too hard to get the plaster out.

I dunno.

Maybe I should see if the 8" prototype version fly's first...

FM.JPG

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Im surprised that the CP is that far back. I have always wanted to do a bomb style shape but never figured out how to do it in Open Rocket.

It is an interesting deal. The full fin set is huge fin area. Also, the entire rear of the rocket is in essence a ogive. Makes for interesting simulation.

I need to run it with motors, but I think it will need to have substantial nose weight.
 
What size is your motor mount, 38mm?

Oh, I meant to describe how I did it.

The nose cone is an elliptical shape adjusted for diameter, length, and approximate mass. All hobby simulators need a body tube, so there as a very short one, and then it transitions to the elliptical tail one.

The fin box to a fair amount of work. Did it with pods in Rocksim. Not sure if Openrocket has that feature/capability.

Then do the inside tubes etc.

I am still tweaking the internals- will report more in a few days once I have a little more time to work on it.
 
Nice project! I was thinking of the same a while ago, but it was meant as a "one day" project.

I did recall seeing very large nesting Easter eggs at Hobby Lobby ("Mega Eggs") and I think Michael's as well (chain craft stores if you're not familiar). I checked and they are a set of 8", 12" and 16"...the largest should fit the smaller end of what you were asking for.

Here's a link to the HL item (no longer available of course...just wait until next Easter for a better chance of just surf the internet for it):
https://www.hobbylobby.com/Seasonal...een-&-Pink-Nested-Plastic-Easter-Eggs/p/35334
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Good luck with the test flight of the 8" build! I'm very interested in hearing how it performs. :)
 
Thanks for the link Ken. Unfortunately, that's the height not the diameter or that would have been perfect for 1/4 scale.

Once again Mark you're my "make crazy things fly safe" go to guy and I really appreciate that. If we made the Javelin 148 with 8 fins mid rocket fly dead straight up. I'm thinking this one can fly. A 1 1/2 lb rocket with another 1 1/2 lbs of lead in the nose, what can go wrong?

Below is the finished prototype. Blowing everything out the back seemed to be the easiest way for recovery. I'm using a simple timer to eject the pod.

Phil

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Hey Guy's,

The builds completed with Mark's Sim showing 1 1/2 lbs in the nose to be stable.

First time I've built a 3 lb rocket where half the weight's in the nose for stability. After adding the nose weight the CG is exactly where Mark's simulation had it. If it's a stable flight, scaling it up to a 1/4 scale 15" in diameter should make for a low and loud A-BOMB, AKA "Fat Man".

fat man sim.JPG
 
Python rocketry could make you a foam and fiberglass shell with a body tube up the center for either a 1/4 or half scale. He has even done fin slots in a custom tail cone.
 
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Python rocketry could make you a foam and fiberglass shell with a body tube up the center for either a 1/4 or half scale. He has even done fin slots in a custom tail cone.

Why only 1/4 or half scale from Python?
Dan from Python can make full or double scale.....We have done 4 projects together and each one has been more fun than the last. Try pythonrocketry.com for any size FATBOY cone.
 
Lol, I think due to material constraints he may max out at 48in diameter. But I could be wrong.
 
Hey Brian,

Thanks for the lead. Emailed Dan and the prices are reasonable for the 15" build. First I'll have to see how the design does in the real world at Bong on the 26th later this month.
 
BTW ,this is a great project and build ,very original to say the least......gotta love it !!

Cheers

Paul T
 
I know.. Holy Thread Resurection!

Did @PSLimo 's Fat Man ever launch?

I've been thinking Fat Man and how to make it fly as a model rocket. I think one key item that is missing on @PSLimo 's build here is the boxing plates that were used on the real Fat Man, see photo below.

My mindsim thinks these plates will add so much drag (& dare I say add Gas Dynamic Stabilization) that the amount of nose weight needed on the rocket would be significantly reduced, if the boxing plates are added. That should make for a more stable rocket, with less weight.

Thoughts?

Fat Man Boxed Fin Arrangement.jpg
 
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